^for cantilever brakes with a fixed cable stop the gap would close before you even apply the brake.
on a cantilever brake the housing goes from the lever (fixed relative to the bike) to a fixed cable stop. the only forces on it when you apply the brake would be friction from the cable and deformation from the cable wanting to go in a straight line, there would be no force of compression.
for caliper and v brakes, however, the housing does experience compression equal to the force of the cable pull.
so it could be argued that these inline barrel adjusters are much safer for traditional cantilevers than for v or caliper brakes.